Gearhead selection

In the design of precision positioning systems, the use of a gearhead with a smaller motor is common. However, the highest levels of precision require either that a zero-backlash gearhead be used, or that a larger motor is used, without a gearhead.

The use of gearheads provides several benefits, such as use of smaller motors and increased servo stiffness. The reflected inertia to a motor through a gearhead is reduced by the square of the gear ratio. Therefore, it is beneficial to use a high ratio gearhead with systems that move high-inertia loads. Not only is the size of the motor reduced, but also the size of the power supply and motor driver. The trade-off is speed vs. torque.

Many small precision motors are designed with maximum speeds of more than 10,000 RPM. It would not be very practical to attempt to drive most leadscrews at such speeds, so there is no real disadvantage to the use of a gearhead with a ratio of 100:1 or higher. A 9000 RPM motor with a 100:1 gearhead can produce 90 RPM. When coupled with a leadscrew having two threads/mm, 90 RPM will produce a linear velocity of 0.75 mm/sec, which is a very useful value for many precision positioning tasks, such as viewing under a microscope.

Perhaps the greatest value of the gearhead in precision positioning is the increased effective resolution of systems using a rotary position encoder on the motor shaft and the increased torque available for small movements. A commonly used rotary encoder produces 2000 encoder states per revolution. When used with a 100:1 gearhead and a half mm pitch leadscrew, each encoder state is equal to 1/400,000 mm, or 2.5 nm. It is important to not expect to be able to use such a system to repeat to an accuracy of 2.5 nanometers, but this does not mean that such high resolutions are useless.

The ability of a system of this type to make small movements is limited by "stiction" friction, which is the additional friction that opposes the onset of motion. After motion is begun, the friction level drops considerably. However, attempting to move in small increments requires a controller capable of providing high torque to begin motion and much lower torque to continue motion. When attempting a move of only a few encoder counts, it is very difficult to provide smooth motion and to avoid overshoot and oscillation. A gearhead increases the torque available to overcome stiction and begin motion. Also, by increasing the effective resolution of the position encoder, the loop gain is increased which further increases the ability to make small steps.

From an economic standpoint, the gearhead will often equal the price of the motor. However, it also reduces the cost of a larger motor, larger power supply and larger motor driver, and perhaps eliminates the need for a higher resolution encoder. Of course it is possible that the existing power supply and motor driver will have sufficient capacity for either motor, but the differences could become substantial in multi-axis systems.